Well, that's the problem.
The only way you can modify the PSP Battlefront 2 is if you have a 1.5 PSP, because 1.5 PSPs can load ISOs. What you have to do is modify the files in the ISO, then load it via a UMD loader.
Otherwise, you can't modify SWBF2 for the PSP at all, which really sucks for me, because I don't own a 1.5 PSP.

Well, that's the problem.
The only way you can modify the PSP Battlefront 2 is if you have a 1.5 PSP, because 1.5 PSPs can load ISOs. What you have to do is modify the files in the ISO, then load it via a UMD loader.
Otherwise, you can't modify SWBF2 for the PSP at all, which really sucks for me, because I don't own a 1.5 PSP.

actually if u have 2.71 or below theyve released a downgrader that will bring u back to 1.5 they work ive used all of them they have a 99% success rate www.psp-hacks.com should have it

That site contains nothing illegal.
For losers like me, I can't downgrade to 1.5 because of the type of PSP I have. When they finally get kernel mode to work on higher versions so I can run ISOs, I'll try to do some SWBF2 PSP modding, too.
I think there might be big differences between PC and PSP preventing things like putting in the levels the PSP is missing, though.
You know what I don't understand is that, in the ISO of Star Wars Battlefront 2 for the PSP, all of the levels are there, including Geonosis and Kamino, and their file sizes are similar to the levels that are actually in the game, suggesting that they're actual levels. However, in the actual SWBF2 game, there's no Geonosis or Kamino (or the other missing levels).
It'd be really annoying if those levels are just hidden for no reason.
Even in the tutorials, there's a picture of Boba Fett standing in Jabba's Palace, even though the Jabba's Palace level isn't in the game. Weird.

I'm sorry to say that, after a lot of testing, I could not get anything from the PC game into the PSP game. Sounds, levels and all that jazz would just crash the game. The only thing I could modify without crashing the game was the mission.lvl, which is mostly useless unless you want to hex edit characters in.
I was really hoping I could increase the amount of AI or put in all of the levels or something. I'm really disappointed.
I managed to replace Polis Massa with Geonosis (I didn't take anything from the PC, Geonosis was already on the PSP), and it appeared as Geonosis on the list of levels (instead of Polis Massa), and when it loaded it had the Geonosis loading screens, but it crashed halfway through loading.

HOLY CRAP! HOLY FRIGGIN' CRAP!
I did it! I successfully replaced Kashyyyk with Geonosis by switching them out in shell.lvl! This very second, I'm a Geonosian killing Episode 2 snipers on the PSP! For some reason, Conquest and CTF won't work, though. Most likely, some vehicles are missing.
You know what this means?
A lot of stuff from the PC and console games is ALREADY on the PSP, but the idiots that made the game didn't allow access to the missing content. How could they miss something like that?
Seeing as how the Geonosis terrain works (vehicles in the sky are missing), and the Geonosians, turrets and Episode 2 clones are all on the PSP, I'm going to see what I can do about the vehicles.
I probably won't be able to do anything, since the vehicles probably aren't on the PSP at all and I've yet to successfully add anything to the PSP from the PC, so Geonosis may have no hope beyond the already successfully unlocked Hunt. We'll see, I guess.
I'll try out some non-Geonosis levels, too, of course.

GEONOSIS: Hunt works without problems. Conquest and CTF don't, probably because the vehicles aren't specified in the mission.lvl, or the vehicles aren't included in the game at all, or something. Hopefully, it's just the mission.lvl. I'd assume that the vehicles are included in the game simply because ALMOST EVERYTHING ELSE THAT WASN'T USED already is.

UTAPAU: ABSOLUTELY FLAWLESS! Barely. The floor will visually disappear on certain close-up polygons, the enemy AI won't use their hero (which ALWAYS happens on the PC and consoles), and the Galactic Civil War era will crash when the player chooses a class. Those are the only problems. The Clone Wars works perfectly aside from the enemies not playing as their hero. The AI navigate the level just fine and use the vehicles like they're supposed to, and there simply are no problems. Why wasn't it included in the game? A few minutes of work and it would have easily become a well-designed, fully playable level.
Utapau has ZERO fog and you can see all the way across the map with little to no expense to the frame rate, showing that the PSP game could have easily handled maximum vision distance on every level, since Utapau has so much detail. Though not seeing very far is the least of the problems I have with the PSP version at this point.

JABBA'S PALACE: Generally the same as Utapau. Works great, except in Control Point 3, characters would fall through the ground for eternity or instantly die, and there were no Gamorrean Guards. Also, Galactic Civil War would work, the enemies would use their hero, and the ground wouldn't disappear. There was still maximum visibility, unlike in the other PSP or console levels (rather, like in the PC levels).
So, this reason wasn't included because of no Gamorrean Guards and a messed-up control point. OH MY GOD, HOW COULD THE CREATORS POSSIBLY FIX THOSE TWO PROBLEMS!?!? Retards.

Coruscant, Death Star, Felucia, and Kamino I couldn't get working, and I can't be entirely sure as to why. One problem is that I can't modify the .LVL files via hex editor to include all of the necessary game types, vehicles/classes and whatnot for missing levels, so that can cause crashing. If I copy the mission.lvl from the PC (the only file I can copy without crashing), the models in the game will get weird and stretchy and nothing will actually be fixed.

The most annoying thing is that all of the original, cut-out, playable levels are on the UMD, the game just prevents access to them. That's so stupid, I can hardly find words for it. Oh, yeah. Stupid. LucasArts' SWBF2 team is stupid.

I'm willing to try to do some hex editing to not only replace, but add new levels by editing mission.lvl. I'd have to get ahold of data_psp lvls:

shell.lvl
mission.lvl

I know the mod tools don't have all the tools necessary to munge PSP maps, but it's extremely possible that the generic munge tools that do things like mission.lvl were built for multiple formats. Feel free to reply or PM me with those files and I'll see what I can do.